We will study the interaction among 1) a brainstem vulnerability, 2) critical age, and 2) homeostatic stressors; three factors that form the Triple Risk Hypothesis for the causation of SIDS. We will evaluate the control of breathing, heart rate and reflex apnea in developing rodents of both genders. Our brainstem vulnerabilities are chosen based on human infant data implicating abnormalities in the medullary serotonergic (5-HT) system, GABA receptor binding, and neurokinin-1 receptor (NK1R) binding, in SIDS (Introductory Overview). The use of rodents allows focus on developmental age, as in 20 postnatal days they mimic late gestation and infancy in humans, as well as genetic and pharmacological manipulation of brainstem vulerabilities. Our additional stressors, chronic intermittent hypoxia, maternal smoke exposure and warm temperatures, are epidemiologically defined risk factors for SIDS. The emphasis is on the interaction of vulnerabilities, age and stressors. Specific aim (SA) 1 focuses on 5-HT in genetically manipulated mice with reduced function of the 5-HT transport protein and with 'silenced'5-HT neurons, i.e., a reduced ability to secrete 5-HT (produced in Project 5). SA 2 focuses on 5-HT but in developing rat pups treated by daily injections of agents to inhibit the 5-HT transport protein or block or stimulate the 5-HT1A receptor, which directly affects function of the 5- HT neuron. Binding for both the 5-HT1A receptor and the 5-HT transport protein is reduced in SIDS brainstems. SA 3 examines the role of the NK1R in developing rat pups via lesions produced by an NK1Rspecific toxin at postnatal day 4 (P4). SAs 4 and 5 will evaluate chronic intermittent hypoxia and maternal smoking, respectively, together with a brainstem vulnerability found most efficacious in SAs 1-3. In each aim, the pups will be tested at P5, P15 and P25 before and after injection of a GABA receptor blocker. We study the control of breathing and heart rate as they are strikingly variable in chance recordings from SIDS cases and inhibitory reflexes, e.g., the laryngeal chemoreflex that induces apnea when receptors around the larynx are stimulated, as they are likely contributors to SIDS. Warm temperatures-thought important in SIDSprolong this apnea and blockade of GABA receptors-abnormal in SIDS-shortens it. Our results, derived from an animal model that directly tests the Triple Risk Hypothesis, will help pinpoint possible mechanisms for SIDS and provide information of use in early detection.